The present invention relates broadly to a waste recycling and metal recovery method. More particularly, the present invention relates to a method for recycling metal-bearing hazardous wastes to recover valuable metals and metal oxides. In the best mode, slags remaining after metal recovery are used to produce mineral wool, and no waste results.
Public awareness of problems associated with the rapid depletion of the earth's natural resources and disposal of industrial wastes has greatly increased in recent times. Such awareness, together with increased economic pressures, the tightening of competition, and government regulation of wastes have forced industrial concerns to take measures to minimize waste. In response, the focus of scientific undertaking in some industries has turned toward recovery and reuse of all commercially useful byproducts of industrial processes.
In the past, little attention was directed to the preservation of limited mineral resources. It was generally deemed more feasible to mine metal ores and to simply dump rich metal-bearing wastes than to salvage usable metals from waste products. This was particularly true in the case of industrial wastes which contained hazardous or toxic materials.
Hazardous industrial wastes were typically "stabilized"and/or captured in some generally non-leachable form with a basic material such as lime or cement. It is required to bury the stabilized materials in designated hazardous waste landfills. One widely accepted disposal method was to incorporate hazardous waste products into a glass-like matrix called slag which was used as a substitute for a natural rock aggregate in cement or asphalt used for paving roads and the like. However, the EPA has declared that no material which is considered hazardous can be applied to the land in any form, whether or not it has been diluted, treated, or otherwise "stabilized". Hence, the older disposal methods have fallen into disfavor. Alternative processes for treating wastes to produce environmentally "safe"products have been proposed.
For example, Frey U.S. Pat. No. 4,432,666 issued on Feb. 21, 1984 describes a process for storing and dumping hazardous wastes. Rostoker, U.S. Pat. No. 4,793,933 issued Dec. 27, 1988 teaches a method for treating metal hydroxide electroplating sludges by fusion of the oxides of the metals into a silica and sodium slag. The Rostoker method relates to earlier EP Leaching standards, and has been proven incapable of achieving minimal-waste recycling. Lynn, U.S. Pat. No. 4,840,671 issued Jun. 20, 1989, relates to the stabilization of EAF dusts for disposal. The latter '671 reference teaches the use of calcium hydroxides as an entrapping agent for toxic cadmium, chromium, and lead constituents. This patent suggests combining various different waste products to be processed to produce "safe" compounds.
However, in view of the general awareness of environmental and health risks, such treatment and disposal techniques are no longer deemed environmentally or economically sound. Moreover, because industry pays a relatively high premium for waste treatment and disposal, it is desirable to provide a commercially viable method for recovering as much usable material as possible. Such methods would be directed to reducing loss of profits and expanding commercial markets. Specifically, it is desired to provide a process which can be carried out with equipment and apparatus already available in the industry.
The United States Environmental Protection Agency (USEPA) has undertaken to classify certain materials for controlled disposal and/or recovery. The list of USEPA-listed hazardous wastes is presently limited, but will undoubtedly be enlarged with time. There are presently a large number of waste products generally recognized as unsafe for conventional disposal which have not yet come under USEPA scrutiny. For example, certain anodizing wastes such as F019 are presently listed but not classified as hazardous; sand used in blasting operations may be contaminated with nickel, chrome, or other metals which are considered toxic; and, baghouse dusts may contain carbon and hazardous materials have no separate classification under the current law. Such wastes are therefore thrown away without meaningful disposal precautions, although they are widely believed to create hazards to the environment. Moreover, their disposal results in unnecessary depletion of existing natural mineral resources.
It is therefore desired to provide a viable method for reclaiming various listed and also non-listed hazardous wastes. Such a method must effectively eliminate waste in order to conserve natural resources and avoid costly liability, for example, under the Resource Conservation and Recovery Acts ("RCRA") or Comprehensive Environmental Response, Compensation, and Liability Act ("CERCLA"). Moreover, such a method must be effective to overcome disadvantages associated with prior sodium-based recovery processes. Such disadvantages include high reagent cost, pH imbalances in slags, low value by-products, production of waste, and undesired volatilization of sodium due to the higher temperatures required to reclaim certain metals (i.e., chromium).
In the prior art known to us, numerous methods are taught for recovering various industry wastes for production of useful products. Such products include furnace fuels, paving aggregates, sealing compounds, and mineral wool.
Mineral wool is a term broadly applied to various related products commonly used for insulation, padding, and the like. In general, mineral wool is a fiberglass-like material composed of very fine, interlaced mineral fibers, somewhat similar in appearance to loose wool. Low-temperature wools are composed primarily of silicates of calcium and aluminum. Mineral wool producers commonly use natural rock or slag. Slag is a term broadly applied to refer to waste products of the primary metal and foundry industries, including the erosion of refractory from the furnace lining, charge impurities, ash from fuel, and fluxes used to clean the furnace and remove impurities. Although metal producers and foundries strive to control the amount of slag, excess slag may result from overfluxing.
Slags are classified as either "acid" (or high silicate) slags or "basic" slags, depending upon the relative quantities of acidic and basic subcomponents. For example, typical acid slags contain between forty and fifty percent (40-50%) silica (SiO.sub.2), an acidic subcomponent, and relatively small quantities of basic components such as oxides of calcium (CaO) and magnesium (MgO). Aluminum oxide (Al.sub.2 O.sub.3), which comes from the furnace lining, ranges from ten to twenty percent (10%-20%) and is considered neutral. A typical basic slag, on the other hand, comprises between twenty-six to forty-seven percent (26%-47%) silica (SiO.sub.2) and between five and twenty percent (5%-20%) aluminum oxide (Al.sub.2 O.sub.3) as the acidic subcomponents, and calcium oxide (CaO) ranging between thirty-two and forty-eight percent (32%-48%) and magnesium oxide (MgO) ranging between seven-tenths and twenty percent (0.7%-20%) as the basic subcomponents. Either magnesium oxide or aluminum may come from the erosion of the furnace lining or magnesium may be added to increase the slag basicity.
Basicity is the tool used to determine metal quality produced from a basic slag. Basicity is calculated as follows: (CaO + MgO) .div. (Al.sub.2 O.sub.3 + SiO.sub.2) Basicity of typical basic slags ranges between 0.93 and 1.9. Generally speaking, metal producers using very high quality metals as charging materials use an acid slag, while metal producers using charge materials which need refining use a basic slag. For best results, mineral wool producers seek slags that can be blended together and melted at relatively low temperatures. Preferably the mineral wool slag will contain no reducible metals, or will be an acid slag, which will eliminate metal buildup in the furnace.
Mineral wool is classified according to the raw materials used in its production. For example, Rock Wool is produced from combinations of natural rocks and/or minerals. Slag Wool comprises a composition of iron, copper, and lead slags typically removed from blast furnaces, and may contain some fluxing materials. Glass Wool is composed principally of silica sand, soda ash, and limestone. Refractory (high-temperature) or "Certa" wool may be made from oxides of aluminum, chromium, zirconium, or titanium and silica sand. Further subclassifications of these products relate to the quality or purity of the wool. For example, slag wool is subclassified for purity according to color; black, gray, and white wools are available. The tool for determining the quality of mineral wool produced from a slag charge is the Acid-to-Base ratio (A:B). The formula for determining A:B is (Al.sub.2 O.sub.3 + SiO.sub.2) .div. (CaO + MgO). In a typical mineral wool cupola slag, A:B ranges between 0.74 and 2.316.
Prior art patents related to the production of mineral wool using various waste products include Gee U.S. Pat. No. 4,822,388 issued Apr. 18, 1989; and Monaghan U.S. Pat. No. 4,486,211 issued on Dec. 4, 1984. The latter-referenced '211 patent discloses a method and apparatus for melting discarded fly ash and spinning it into mineral wool. However, none of the prior art known to us teaches viable methods for recycling listed hazardous materials such as Chromium, Nickel, Cadmium, Zinc, Copper, Iron, and Lead Oxides or hydroxides into pure metals or alloys while producing mineral wools from Aluminum, Zirconium, and Titanium oxides
Other relevant prior art patents known to us relate to methods for treatment, recovery, and recycling. For example, Allen, U.S. Pat. No. 3,870,507, issued Mar. 11, 1975 is directed to a method for forming briquettes from steel mill wastes such as steel and iron dust, mill scale, and iron oxides with an organic binder to reduce slags formed during recycling. The resulting iron oxide briquettes are recycled by being fed into the production furnaces with new materials in the steel-making process. Fukuoka U.S. Pat. No. 4,004,918 issued on Jan. 25, 1977, teaches a method for treating certain wastes resulting from stainless steel operations. Briquettes are formed from the dust and scale from stainless steel ovens combined with organic and inorganic binders. The briquettes are returned to the existing electric arc furnace, and usable metals are extracted for further use in making stainless steel.
Stephens U.S. Pat. No. 4,396,423 issued Aug. 2, 1983 and related U.S. Pat. No. 4,053,301 issued Oct., 1977 relate to a process for recovery of iron carbide and zinc metals from BOF dusts of the steel-making process. The Stephens system reduces the dust wastes within a fluidized bed reactor in the presence of carbon, recovers zinc by vaporization, and produces iron carbide and gangue, a worthless rock or matter in which metals are contained.
U.S. Pat. Nos. 4,758,268 issued Jul. 19, 1988 and 4,836,847 issued Jun. 6, 1989 to Bishop disclose apparatus and processes for reclaiming metals from electric arc furnace and BOF dusts. The systems described therein are directed to providing recovery of metals from EAF wastes in a reducing environment. In the method, carbon is added to the molded briquettes to reduce the iron and zinc content of the waste. However, the process is incapable of producing a slag suitable for use in the production of mineral wool, since these processes attempt to minimize slags to less than 8%. Moreover, the Bishop system is specifically indicated to be unsuited for rotary kilns, shaft furnaces, retorts, and fluidized bed furnaces
For purposes of clarity, various terms familiar to those skilled in the art and commonly used in the industry are applied herein and shall be clarified as necessary in context. Various hazardous wastes specifically identified shall be referred to by their standard USEPA designations, as for example, K061 (electric arc furnace dust).